How did scientists get the idea that the appendix was useless?
There had been a lot of discussion about what the appendix might do as a function, whether it served a function, prior to [Charles] Darwin’s time. The [fact] that we can live without it does provide some support for the idea that it’s vestigial and it doesn’t really do anything. And so Darwin’s interpretation of it as a vestige was reasonable at the time, given the information that he had.
But now with modern technology, we can see things like the microanatomy and the biofilms in the appendix, and we have a better understanding of what it is and what it’s doing.
How has the appendix evolved over time?
If you map the distribution of appendices across a phylogeny — a tree of mammal life — you can interpret that the appendix has actually evolved independently. It has appeared independently multiple times throughout mammalian evolution. So that is evidence that it must serve some adaptive function. It’s unlikely that the same type of structure would keep appearing if it wasn’t serving some beneficial role.